Polarized light source device

ABSTRACT

A polarized light source configured for use in display device and backlight unit is described. The polarized light source comprising: at least one optically active structure comprising a plurality of nanorods configured to emit light of one or more wavelengths in response to exciting pumping field, said plurality of nanorods comprising nanorods aligned with a predetermined alignment axis so as to produce a desired polarization direction of the emitted light; and a light directing assembly comprising one or more optical elements in optical path of light emitted from the light emitting structure, said light directing assembly being configured to enhance output of the emitted light from the emitting structure while substantially maintaining the polarization of the emitted light passing therethrough. Preferably, layers associated with the polarized light source are aligned with parallel principal axes.

TECHNOLOGICAL FIELD

This invention is generally in the field of light sources, and relatesto a polarized light source device, particularly useful as a backlightunit in a display device.

BACKGROUND ART

References considered to be relevant as background to the presentlydisclosed subject matter are listed below:

-   -   1. US 2011/0299001    -   2. WO2012/059931    -   3. US 2013/0026506    -   4. U.S. Pat. No. 8,471,969    -   5. US 2012/0113672    -   6. WO2012/021643    -   7. WO2010/0155749    -   8. U.S. Pat. No. 5,825,543,    -   9. WO 2008/027936    -   10. U.S. Pat. No. 7,278,775,    -   11. U.S. Pat. No. 8,033,674    -   12. “A polarized laser backlight using a zero-zero-birefringence        polymer for liquid crystal displays”, Takahiro Kurashima, Koichi        Sakuma, Takayuki Arai, Akihiro Tagaya, Yasuhiro Koike, Optical        Review, Volume 19, Issue 6, pp. 415-418 (2012)    -   13. U.S. Pat. No. 6,746,130    -   14. US 2008/0285255    -   15. U.S. Pat. No. 6,111,696    -   16. “Novel wide viewing liquid crystal display with improved        off-axis image quality in a twisted nematic configuration”,        Seongmo Hwang et al., Samsung Electronics Co. Ltd, Optical        Engineering 48(11), p. 114001 (November 2009).

Acknowledgement of the above references herein is not to be inferred asmeaning that these are in any way relevant to the patentability of thepresently disclosed subject matter.

BACKGROUND

Flat-screen displays are widely used in various devices such ascomputers, mobile phones and television sets. Liquid Crystal displays(LCDs) present a major part of flat-screen displays. LCDs utilize backilluminated LC panel, where the LC panel is typically a multi-layerstructure and includes a liquid crystal layer configured for modulatingtransmission of light towards the viewers. Light transmission througheach pixel is controlled by changing the polarization state of theliquid crystal.

LCDs utilize polarized backlight illumination. Current color LCDdisplays use a backlight unit emitting non-polarized, generally white,light (i.e. polychromatic light that has no specific polarization), anda polarizer in the optical path of the emitted non-polarized light.Thus, selection of a particular polarization comes at the cost of energyloss: exemplarily, more than 50% of light emitted by the light sourcemay be lost due to the light passage through the polarizer. This problemis significant for LCD displays, where energy saving is a crucialfactor. The problem is further intensified in portable devices (laptops,cell-phones, tablets, etc.) where battery life and increased backlightbrightness are important factors.

Backlight illuminators based on emission properties of nanostructureshave been developed and are described for example in WO 2012/059931,assigned to the assignee of the present application. According to thistechnique, an optically active structure is provided, which may be usedas colour polarized light source for displays. The structure comprisesat least two different groups of optically active nanorods differingfrom one another in at least one of wavelength and polarization of lightemitted thereby in response to a pumping field. The nanorods of the samegroup are homogeneously aligned with a certain axis of alignment.

GENERAL DESCRIPTION

There is a need in the art in a novel approach for configuring apolarized light source based on emitting nanostructures for maximizingpolarization of light interacting with the nanostructures as well as formaximizing output efficiency of the polarized light.

The present invention provides a polarized light source including one ormore optically active layers/films comprising aligned nanorods ofselected sizes to emit light of certain polarization in response toexciting field (optical or electrical pumping field); and a polarizationpreserving light (directing/guiding/extracting) assembly. The latter maybe integral with the optically active layers/films, or may be externalthereto.

It should be noted that generally polarization of light should bepreserved for light passing through or interacting with various opticalelements commonly used inside an LCD backlight unit. Such opticalelements may generally include reflectors, lightguides, diffusers,brightness enhancement film, etc. Thus, the structure and composition ofsuch a polarized light source are to be carefully designed to maintainthe desired polarization of output light so the light incident on aliquid crystal layer (e.g. pixel matrix) is kept desirably polarized. Tothis end, the present invention provides various backlight film stackconfigurations in order to obtain polarized emission based backlightingby integrating the polarized light source together with complementaryoptical elements of the backlight unit (BLU).

Further, it should be noted that cost reduction, lower power consumptionand improved viewing experience for users are important considerationsin the design of LCDs. The polarized light source of the presentinvention based on polarized emission backlighting, provides for lowerpower consumption and/or brighter screens. To this end, efficient lightemission provided by arrays of aligned nanostructures is accompaniedwith efficient light extraction from the emitting layer, whilemaintaining polarization in order to minimize losses. The polarizedemission originates from the polarized light source, which includesaligned arrays of anisotropic nanostructures, and in particularcolloidal semiconductor nanorods.

For the purposes of a backlight unit, a polarized light source maycontain a homogeneous mixture of at least two groups of optically activenanorods, differing from each other in the emission wavelength.Preferably, the layer of such mixture contains green (central wavelengthin the range of 520-560 nm) and red (central wavelength in the range of600-640 nm) emitting nanorods. The polarized light source may be excitedby blue light from a pumping light source e.g. LEDs (central wavelength440-460 nm, preferably 450 nm). The concentration of emitting nanorodsin the active layer is adjusted to allow part of the incident pumpinglight to be transferred through the layer, while exciting the nanorodsin the layer to emit the complementary green and red light needed toproduce white light. In some configurations, the layer may additionallyinclude blue emitting nanorods, and be configured to emit light inresponse to ultra-violet pumping light (central wavelength 350-405 nm).In such configuration, the concentration of nanorods is selected toabsorb higher portion of the pumping light, and an additional filter maybe used for blocking the remaining portion of the pumping light.

It should be noted that emission wavelength of the nanorods is generallydetermined in accordance with size, geometry of the nanorods, as well asmaterial composition of the nanorods. It should also be noted thatgenerally the suitable pumping wavelength range is determined byappropriate selection of material composition of the nanorods.

In all embodiments of the current invention, the polarized light sourceincludes an optically active structure containing such nanorods (orother anisotropic nanostructures) arranged in one or more layers andbeing homogeneously oriented along a distinct axis of alignment. In someembodiments, the nanorods can be embedded in a matrix medium, such as apolymer film, adhesives matrix (epoxy), silicone matrices or glass. Insome other embodiments, the nanorods can be located/deposited on thesurface of a substrate. The alignment of the nanorods along a distinctaxis (meaning that the elongated axis is parallel to the alignment axis)can be achieved by several suitable techniques, such as deposition onspecial chemically or physically treated substrates (for examplepatterned groove-like pattern), applying electric fields, polymer filmstretching or applying mechanical force, or any other known suitabletechnique of inducing preferred alignment. The alignment of nanorods (byany of alignment techniques) leads to emission of light which issubstantially polarized along a distinct axis (also referred to as the“polarization axis”).

As indicated above, in some embodiments the polarized light sourceincludes aligned nanorods embedded in a polymer matrix. Examples of suchpolymers include polymers of Polyacrylamide, polymers of Polyacrylicacids, cyclic olefin copolymers, polymers of Polyacrylonitrile, polymersof Polyaniline, polymers of Polybenzophenon, polymers of poly(methylmathacrylate), polymers of polyesther, silicone polymers, polymers ofPolybisphenol, polymers of Polybutadiene, polymers ofPolydimethylsiloxane, polymers of Polyethylene, polymers ofPolyisobutylene, polymers of Polypropylene, polymers of Polystyrene andPolyvinyl polymers (Polyvinyl alcohol, polyvinyl butyral). The thicknessof the optically active structure in these cases may exemplarily rangebetween 1 μm to 1000 μm (microns, micrometer), or between 1 μm and 300μm, or preferably between 5 μm and 200 μm. Additionally, the opticallyactive structure may be configured by a substrate carrying plurality ofnanorods on a surface thereof. It should be noted that in suchconfiguration, the optically active structure may be of thicknessbetween 0.05 μm to 5 μm and preferably 0.1 μm to 1 μm.

The polarized light source of the present invention containing suchoptically active medium with aligned nanorods is generally suitable tobe incorporated into the backlight unit of any type of LCD devices. SuchLCD device may include a bottom polarizer, located between the backlightunit and an LC layer/structure. In this case the polarized light sourceis placed in the backlight unit in a manner allowing increasedtransmittance of polarized emitted light through the bottom polarizer.This can be achieved by fixing the orientation of the “polarizationaxis” to be substantially parallel to the transmittance axis of thepolarizer so that significantly higher fraction of the polarized lightemitted from the light source is transmitted through the polarizer, ascompared with the typical non-polarized backlight unit configuration.

As indicated above, polarization maintaining of light passing throughvarious optical elements commonly used inside a backlight unit of an LCD(e.g. reflector, lightguides, diffusers, brightness enhancement film,etc.) is an open challenge. The present invention provides variousbacklight film stack configurations required in order to obtainpolarized emission based backlighting, using a polarized emissive filmtogether with other complementary optical elements.

More specifically, the invention provides light extraction methods,which maintain polarization of light extracted from the optically activemedium (e.g. film); a light redirection film and other backlight opticalelements (e.g. brightness enhancement films (BEF), diffusers,lightguides, reflectors, microlens films, reverse prisms films)configured especially to maintain polarization; edge illuminationpolarized nanorods film; direct bonding of an optically active film to alightguide; as well as direct bonding of an optically active film to thebottom polarizer of an LC cell.

According to the light extraction methods of the invention, apolarization dependent scattering film is prepared based on mixingaligned emitting nanorods together with birefringent particles/polymers.This is a combination of an anisotropic polarized emissive material(nanorods) together with such anisotropic polarized scattering elements,configured to preserve the polarization of the emitted and extractedlight.

In some embodiments, an additional diffusive layer may be used, locatedin optical path of light emitted from the optically active structure.The diffusive layer may be used for providing uniform illumination andcompensate for variation of nanorods density and/or intensitydistribution of the pumping light. The diffusive layer is preferablyconfigured with haze factor in the range of 50% to 95%. Additionally oralternatively, the diffusive layer may be attached to the surface of theoptically active structure. In these configuration the diffusive layeris configured to increase uniformity of pumping light, when back pumpingis used, and/or to increase light extraction efficiency by varyingdirectionality of light emitted from within the layer.

As indicated above, in some embodiments, the edge illumination ofpolarized nanorods film is used, and/or a polarization preservinglightguide and/or a reverse prism film to maintain the polarization oflight emitted by the nanorods.

Thus, according to a broad aspect of the invention, there is provided apolarized light source comprising:

-   -   at least one optically active structure comprising a plurality        of nanorods configured to emit light of one or more wavelengths        in response to exciting pumping field, said plurality of        nanorods comprising nanorods aligned with a predetermined        alignment axis so as to produce a desired polarization direction        of the emitted light; and    -   a light directing assembly comprising one or more redirecting        optical elements in optical path of light emitted from the light        emitting structure, each of said one or more redirecting optical        elements being configured to affect direction of propagation of        a light interacting with said redirecting optical element while        substantially not affecting polarization of said light, said        light directing assembly being thereby configured to enhance        output of the emitted light from the emitting structure by        optimizing polarization (e.g.

direction and/or state of polarization) and intensity distribution ofoutput light.

In some embodiments, the optically active structure comprises a hostmatrix structure in which the emitting nanorods are embedded. The matrixstructure is configured to enhance the output of the emitted lighttherefrom while substantially maintaining the polarizationcharacteristics of the emitted light while passing through the matrixstructure. The host matrix structure may for example comprise a hostmaterial and scattering particles mixed with the nanorods, and theconfiguration is such that the host material and the scatteringparticles have different birefringence. For example, the host material(matrix) has zero or relatively low birefringence as compared to adifference between the fast and slow axes birefringence of thescattering particles; or the scattering particles have zero orrelatively low birefringence as compared to a difference between thefast and slow axes birefringence of the host material. In thisconnection, the term “low birefringence” is used to indicate a minimaldifference between the refractive indices of the different principalaxes (slow and fast axes) of the material composition of the hostmatrix, such that the accumulative induced retardation is minimal asfollows.

It should be noted that generally the polarization direction of lightpassing through a certain material is determined by the retardationinduced by the material. More specifically, the retardation is definedas the difference in optical path of light components havingpolarization along the fast and slow axes of the material, i.e. (Δn*d)where Δn=|n_(o)−n_(e)|, and d is the thickness of the layer. Generallyaccording to the present invention, retardation induced by the opticallyactive structure does not exceed 100 nm and is preferably below 50 nm,and even more preferably the retardation of the optically activestructure may be below 25 nm.

Generally, the scattering particles may be configured (e.g. orientedwith respect to alignment axis of said nanorods in case of anisotropicparticles) to provide refractive index matching for light componentshaving polarization orientation perpendicular to said alignment axiswhile providing mismatch in refractive index for light componentspolarized along said alignment axis.

In some embodiments, the light directing assembly comprises a diffuserlocated in an optical path of pumping light propagating towards theemitting structure.

The light directing assembly may comprise a diffuser located in anoptical path of the emitted light propagating from the emittingstructure.

Generally, either the diffuser in optical path of pumping light towardsthe optically active structure and the diffusive layer in optical pathof light emitted from the optically active structure when used arepreferably configured with haze factor between 50% to 95% and morepreferably between 80% and 95%. Additionally or alternatively, thesediffusers may be attached to the optically active structure from eitherside thereof.

In some embodiments, the polarized light includes an optical pumpingassembly producing the pumping light. The optical pumping assembly isgenerally configured to generate and direct pumping light for excitingsaid optically active structure. The optically active structure and theoptical pumping assembly may be configured such that the pumping lightenters the emitting structure along an axis substantially parallel to alight output direction of the polarized light source (backlight pumpinglight assembly); or may be configured such that the pumping light entersthe emitting structure along an axis substantially perpendicular to alight output direction of the polarized light source. Thus, the pumpingassembly may be configured to provide either direct backlight pumpingand/or side illumination pumping (edge lit backlight unit).

The optical pumping structure may include a lightguide for receiving apumping light from a pumping source and directing it towards theemitting structure.

As indicated above, the light directing assembly generally comprises oneor more optical elements for redirecting light emerging from theemitting structure. The light redirecting optical element may beconfigured for directing light components emitted from the opticallyactive structure to thereby optimized polarization and intensitydistribution of light output from the device. The light redirectingoptical elements may preferably comprise optical elements such as: lightrecycling optical elements, reflective layer and diffusive layer. Thelight redirecting optical elements may also be configured to be alignedsuch that a principal axis thereof is parallel to axis of alignment ofthe nanorods. Thus the fast or slow birefringence axes are substantiallyparallel or substantially perpendicular to the alignment axis of theemitting nanorods thereby preserving the polarization characteristics ofthe emitted light. Additionally or alternatively, the light redirectingoptical elements may be configured with reduced retardation, preferablybelow 100 nm, and more preferably below 50 nm.

According to yet some embodiments of the invention, the optically activestructure may comprise a protective layer structure configured toprovide at least one of mechanical support, strain protection andchemical protection to said optically active structure. Said protectivelayer structure may be configured with at least one of the followingconfigurations: said protective layer structure is oriented such that aprincipal axis thereof is parallel to said axis of alignment of saidnanorods; and said protective layer structure is configured to induceretardation below 100 nm.

According to yet some embodiments of the invention, the optically activestructure may comprise a protective layer configured to provide at leastone of mechanical support or strain protection and chemical protectionto said optically active structure; said protective layer beingconfigured to induce retardation below 100 nm.

Generally, the protective layer may be configured as at least one of thefollowing: a transparent encapsulating layer, barrier layer andmechanical support film.

According to some embodiments, the present invention provides apolarized light source as indicated above; wherein said at least oneoptically active structure and said light directing assembly definingtogether an optical stack arrangement comprising at least two layersoriented such that principal axes of birefringence of said at least twolayers are parallel to one another. The optical stack therebysubstantially preserving predetermined polarization properties of lightemitted from the optically active structure and passing through saidoptical stack.

According to yet some embodiments, the present invention provides adisplay device comprising the polarized light source described above anda spatial modulating unit configured for varying spatial distribution ofoutput light of the polarized light source to thereby provide adisplayed image.

According to one other broad aspect of the invention, there is providedan optical stack arrangement, suitable for use in an illuminationdevice. The optical stack arrangement comprises one or more layersoriented such that principal axes of birefringence of said one or morelayers are parallel to each other; said optical stack therebysubstantially preserving predetermined polarization properties of lightpassing therethrough. The optical stack may further comprise at leastone diffusive layer configured for increasing uniformity of lightpassing therethrough.

The optical stack may comprise an optically active structure comprisinga plurality of nanorods aligned along a predetermined axis beingparallel to said principal axes of the optical stack.

According to one other broad aspect of the invention, there is provideda backlight unit for use in a display device, the backlight unitcomprising a pumping light source and the optical stack as describedabove.

According to one other broad aspect of the invention, there is provideda display device comprising the above described backlight unit and aspatial modulating unit (for example liquid crystal cell) configured forvarying spatial distribution of emitted light to thereby provide adisplayed image.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to better understand the subject matter that is disclosedherein and to exemplify how it may be carried out in practice,embodiments will now be described, by way of non-limiting example only,with reference to the accompanying drawings, in which:

FIG. 1 illustrates schematically a polarized light source device basedon a diffuser optically attached to the optically active layer whichcontains the nanorods;

FIGS. 2 and 3 show two examples respectively of the configuration of anoptically active structure of the invention, wherein aligned emittednanorods are embedded in host matrix and are mixed with scatteringparticles;

FIGS. 4A to 4C schematically illustrate the principles of usingscattering particles mixed with the aligned nanorods, where FIG. 4Aillustrates the light propagation scheme for external light incidenceupon a film that contains no scatterers and the cone of light beams withangles smaller than the critical angle for Total Internal Reflection,and FIGS. 4B and 4C illustrate light propagation scheme for light raysemitted by nanorods inside the optically active structure of the presentinvention;

FIGS. 5A and 5B illustrate schemes of interaction of light emitted froma single nanorod embedded in the optically active structure with alarger anisotropic scatterer aligned substantially parallel to thenanorods' long axis (FIG. 5A) and perpendicular to the nanorods' longaxis (FIG. 5B);

FIG. 6 illustrates an example of an LCD device utilizing the backlightunit configured according to the invention;

FIG. 7 shows a schematic drawing of the polarized backlight unit usingnanorods films as color converters utilizing the direct attachment ofthe optically active structure to the lightguide;

FIG. 8 exemplifies the configuration where the backlight unit of thepresent invention utilizes direct attachment of the optically activestructure to a liquid crystal cell;

FIG. 9 shows schematically a polarized backlight unit of the inventionwhere a nanorods-containing strip is placed on the edge of a lightguide;

FIGS. 10A and 10B illustrate examples of a BEF suitable for use in thepresent invention;

FIG. 11 exemplifies a Light Re-directing Optic Elements (LROE) suitablefor use in the invention, where an external layer with birefringentproperties is used that compensates the LROE birefringence for lightpassing through the combination of the two films;

FIG. 12 illustrates schematically an LCD device having an active lightmodulation module and a backlight unit of the invention;

FIG. 13 shows experimental results of emitted light intensity for anexample of a polarized light source with aligned red-emitting nanorods,wherein the intensity of blue pumping light transmitted by the polarizedlight source and red light emitted by the aligned nanorods weremeasured, with a polarizer's preferred plane of polarization beingaligned parallel to the nanorods for the case of nanorod containinglayer only (left side in the figure), and for the case of using aspecial BEF with birefringence axis aligned with the nanorods' alignmentaxis in front of the nanorod containing layer (right side of thefigure); and

FIG. 14 shows experimental results of emitted light intensity for anexample of a polarized light source utilizing a mixture of alignedred-emitting and green-emitting nanorods, where the sum intensity forRed and Green light was measured for the case with a polarizer'spreferred plane of polarization being aligned parallel to the nanorods'alignment axis, and a BEF film composed of different polymer materials:PET, PC and PMMA.

DETAILED DESCRIPTION OF EMBODIMENTS

The present invention provides a novel polarized light source, which isparticularly suitable for use in a backlight unit of a LCD device, and aLCD device utilizing such polarized light source. The polarized lightsource of the invention includes an optically active structure includingat least one layer of aligned emitting nanostructures excitable by apumping field (optical and/or electrical) and polarization preservingassembly, which includes one or more elements integral with (embeddedin) the optically active structure and/or one or more elements externalto the optically active structure.

In the description below, the optically active structure is forsimplicity termed “film”, but it should be understood that suchstructure may be a single- or multi-layer structure. It should also benoted that such a film may be formed by layers with nanostructuresemitting light of different colors. Also, such a film may includenanomaterial structure(s)/layer attached to another layer providing asubstrate of the emitting nanomaterial. More specifically, according tothe present invention the optically active structure may be configuredfrom a matrix (e.g. polymeric matrix) in which a plurality of emittingnanorods is embedded or a substrate on which the plurality of nanorodsis deposited. Also, the optically active structure may be configured oftwo or more layers including one or more matrix layers carrying nanorodsemitting light at different wavelength ranges. Additionally, such bi- ormulti-layer configuration may be used for nanorods deposited on asubstrate, i.e. bi- or multi-layers including two or more layers ofsubstrate on which nanorods are deposited. Generally, a combination ofnanorods embedded in a matrix and nanorods deposited on a substrate mayalso be used.

Also, the term barrier or protection layer used herein refers to a layeror coating material applied onto the optically active structure andconfigured to provide mechanical support and/or chemical protection tothe optically active structure. In this connection it should be notedthat in embodiments where the optically active structure includesnanorods deposited on a substrate, a protection layer is preferablyapplied onto the nanorods to prevent oxidization, wear and otherchemical damage to the nanorods. In addition to the protective layer, aswell as in embodiments utilizing nanorods embedded in a matrix, abarrier layer or coating material may also be applied onto the opticallyactive structure. Such barrier layer/coating may be applied at eitherone or both sides of the optically active structure. In some casesmultiple protection layers can be used (e.g. a layer to protect fromoxygen, a layer to protect from water)

Also, in the description below, the elements of the polarizationpreserving assembly are at times referred to as light extracting and/or“light enhancement and/or light recycling. It should however be notedthat, generally, such elements form a light directing assembly that isconfigured to enhance output of the emitted polarized light from theemitting structure while substantially maintaining the polarizationcharacteristics of light interacting with elements of the lightdirecting assembly. In this connection, it should be understood that thepolarization characteristics to be maintained actually include both thepolarization and polarization direction. More specifically, bothpolarized light intensity as well as the direction of polarizationshould be at least partially maintained. Further, it should beunderstood that such light directing assembly may be incorporated in theoptically active structure, or be formed as an independent/separateassembly, or some of the elements of the light directing assembly may beincorporated in the optically active structure and some be externalthereto.

Thus, in the polarized light source device of the invention, a film(constituting an optically active structure including one or more layersof aligned emitting nanostructures) may include various light directingelements on one or both surfaces of the structure (film). Such lightdirecting elements may include one or more of the following: prisms,pyramids, microlens, microlens array, metallic reflecting surface, etc.,which are configured to enhance the output of the emitted polarizedlight from the emitting structure while substantially maintaining thepolarization and polarization direction of light emitted from the film.The film may be configured with various structures/interfaces havingselected different refractive indices assisting in the light directionand polarization preservation (e.g. gradient-index lens-likestructures). The light directing assembly may include one or morepolarization preserving diffuser films attached to the optically activestructure from one or more sides thereof. The optically active film mayadditionally or alternatively include various combinations of lightscattering particles of regular and non-regular shape that may enhancethe polarization properties.

The optically active structure may include a host matrix in which thealigned emitting nanorods are embedded. It should be noted that in suchconfiguration, light emitted from the embedded nanorods might besubstantially trapped within the matrix due to total internal reflection(TIR) at the matrix/surrounding (e.g. air) interface. In order toincrease the efficiency of the polarized light source, light extractiontechniques can be used for efficient out-coupling of the emitted lightas well as for suppressing the waveguide behavior of the matrix.

It is generally known to mix high concentrated (up to 1% wt) scatteringparticles (such as TiO2, SiO2, BaTiO3, BaSO4, and ZnO) within theemitting structure. However, such highly diffusive matrices depolarizelight due to the isotropic scattering characteristics. Hence, this knowntechnique by itself is less preferable in a light source of theinvention, where the emitting structure is to emit polarized light.

Reference is made to FIG. 1 exemplifying the technique of the inventionfor efficiently extracting and directing light from the polarized lightemitting structure, i.e. maximizing the extraction of emitted polarizedlight from the emitting structure, while preserving the polarizationcharacteristics of the emitted light. In this example, polarizationpreserving light directing of light from the nanorods layer isfacilitated by using a diffusive layer attached (bonded) to a backside(light input side) of the optically active structure, to cause lightemitted from nanorods within the structure to brake conditions for totalinternal reflection to thereby cause light to escape the opticallyactive structure (film) through a surface of the structure opposite tothat of the diffuser. It should be noted that such diffusive layer mayadditionally or alternatively be attached to front side (light outputside) of the optically active structure. As shown, a light source device100 includes an optically active structure 102 in the form of alignednanorods film, which is associated with a pumping unit 104. In thisexample, the pumping unit 104 includes an optical pumping source 106,e.g. a blue LED, optically coupled to an edge of a lightguide 108.Further provided in the device 100 is diffusive layer 110 bonded to aback surface of the structure/film 102, by which it faces the lightoutput surface of the lightguide 108. Pumping light L_(p) is thusout-coupled from the lightguide 108 and into the nanorods film 102 bythe diffusive layer 110. The diffusive layer 110 eliminates totalinternal reflection and increases the emission L_(em) (e.g. red andgreen emission) from the nanorods film 102. The emitted light L_(em)from the nanorods film 102 is then passed through another polarizationpreserving diffuser 112 to improve brightness uniformity thus optimizingintensity distribution of the emitted light.

Thus, in this example, the light directing assembly may include adiffusive layer, e.g. having haze range between 50% and 95% and morepreferably between 80% and 95%. The diffusive layer is configured andoperable to cause light scattering while at least partially maintainpolarization of light components. It should be noted that the diffusivelayer is preferably configured to fully maintain the polarization ofscattered light. This diffusive layer eliminates, or at leastsignificantly reduces the effect of total internal reflection andincreases the forward emission of light from the polarized light source(e.g. both the pumping light and the optically active structures).Generally the diffuser may be configured of several diffusing layerscascaded one on the other, and/or diffusing layers attached to back andfront sides of the optically active film. For example, such apolarization maintaining diffusive layer can be based on acrylicpolymers, like those used in “Scotch® Magic™ Tape” commerciallyavailable from 3M. It should however be noted that the diffusive layermay be made of any suitable transparent or semi transparent materialconfigured to preserve polarization of diffused light. Such diffusivelayer may contain one or more rubber layers, silicone layers or modifiedacrylic based polymer layers.

Generally, the invention may also utilize the polarization preservinglight directing assembly by obtaining polarized scattering, for whichlight in the polarization axis is scattered instead of being trapped bytotal internal reflection within the optically active unit. Variousconfigurations for achieving polarized scattering are known in the art.In the present invention, the anisotropic emitting nanoparticles, suchas nanorods used for providing polarized optical emission, may beincorporated into a scattering medium (film) having anisotropicpolarization scattering properties, designed to scatter the polarizedemitted light while maintaining polarization. Generally, scatteringproperties of the medium, or of scattering particles embedded therein,may be tailored such that light components of certain selectedpolarization state undergo scattering while the scattering medium issubstantially invisible/transparent to light components of theorthogonal polarization state.

In some embodiments the scattering medium is configured in the form ofnon-birefringent scatterers embedded in a birefringent film (matrix). Insome other embodiments, birefringent scatterers are embedded in anon-birefringent film-matrix are used. In this connection, it should beunderstood that, generally, both the scatterers and the matrix can bebirefringent at some extent, but with a certain difference in thebirefringence thereof. In other words, either the scatterers or thematrix material is considered as birefringent phase and the other isconsidered as a non-birefringent phase, where the non-birefringent phasehas a relatively low birefringence as compared to the birefringentphase, i.e. as compared to a birefringence difference between the twoaxes of the birefringent phase. For example, if the matrix hasrefractive indices n_(1e) and n_(1o) and the scatterers have refractiveindices n_(2e) and n_(2o), the scattering medium is preferablyconfigured such that at least one of Δn_(e)=|n_(1e)−n_(2e)| andΔn_(o)=|n_(1o)−n_(2o)| is much smaller than the other.

According to some embodiments, the polarized light source may include anoptically active structure/film 102 including a mixture of alignednanorods and scattering particles dispersed in a birefringent hostpolymer matrix. This is exemplified in FIGS. 2 and 3. In these examples,the scattering particles are configured for isotropic and polarizationindependent scattering (FIG. 2) or configured for selective scatteringutilizing appropriate birefringence characteristics (FIG. 3). In thisconfiguration as shown in FIG. 2, an optically active structure 102 isconfigured as a polarized scattering film made from birefringent hostpolymer matrix 140 in which aligned nanorods 130 are embedded in amixture with scattering particles 120. The scattering particles 120 aregenerally isotropic and non-birefringent. Generally, the scatteringparticles (scatterers) may be configured of any shape, e.g. thescatterers may be spherical, elongated or of any geometrical shape. Therefractive indices of the host polymer matrix 140 and of the scatterers120 are selected to provide matching refractive indices for onepolarization direction, while providing certain refractive indexmismatch along the other axis. More specifically, the polymer matrix 140has refractive index n_(o) and n_(e) for the ordinary and extra-ordinaryaxes and is preferably configured such that the axis of alignment of thenanorods 130 is parallel to one of the ordinary and extra-ordinary axesof the matrix 140. Additionally, the scatterers 130 are configured froma material having refractive index n_(s), which is selected to be asclose as possible, and preferably equal, to at least on the n_(o) andn_(e) of the matrix 140. This provides that along one axis of lightpolarization (e.g. the extra-ordinary axis being x-axis) there is arefractive index mismatch causing scattering of light, while along theorthogonal axis the refractive indices match and eliminate, or at leastsignificantly reduce, the scattering. This refractive index mismatch isconfigured to provide efficient scattering of polarized light emittedfrom the nanorods, such that the emitted light is out-coupled from thematrix 140. Along the orthogonal axis, e.g. y-axis, the refractiveindices of the matrix and scatterers are substantially matched toeliminate or at least significantly reduce scattering of light in thecorresponding polarization state. Thus, light components of thepreferred polarization (e.g. along the x-axis) are out-coupled away fromthe matrix 140, and light components of the orthogonal polarizationdirection are generally transmitted through the matrix or trappedtherein by total internal reflection. It should be noted thatbirefringence of the host polymer matrix 140 and the alignment of thenanorods 130 may for example be obtained simultaneously by mechanicalstretching of host polymer matrix 140. Thus, as indicated above, in thisconfiguration, the refractive indices of the scattering particles 120 ischosen so that the continuous matrix 140 and the scattering particles120 are substantially mismatched in one axis (the polarization axis) andsubstantially matched in the orthogonal axis. This configuration leadsto increased scattering of light along the polarization axis of thealigned nanorods which leads to preferential light extraction of thedesired polarization. The orthogonal polarization is transmitted throughthe scattering phase and trapped by TIR. The indices of refraction ofthe host matrix and the dispersed particles may typically differ by lessthan 0.03 in the match direction, more preferably less than 0.02, mostpreferably less than 0.01. The indices of refraction of the host and thedispersed phase preferably differ in the mismatch direction by at least0.03, more preferably at least 0.1 most preferably at least 0.2.

In another example, the nanorods may be mixed in a non-birefringent hostmatrix together with aligned anisotropic scatterers, which exhibitbirefringence effects. This is exemplified in FIG. 3 showing anoptically active film 102 made of a polymeric host matrix 140 thatincludes aligned nanorods 130 and anisotropic scattering particles 120.Here, the optically active structure is configured as a polarizedscattering film 102, and includes a host polymer matrix 140 withemitting nanorods 130 embedded therein being mixed with anisotropicparticles 120. Both the anisotropic scatterers and the nanorods arealigned in the host matrix 140 along a preferred axis, e.g. the x-axis.More specifically, the longer axis of anisotropy of the scatterers isparallel to the polarization axis of the nanorods. The anisotropicparticles 120 are selected to have refractive indices along thecorresponding ordinary and extra-ordinary axes, such that along one axisthe refractive index matches that of the polymer matrix 140 and alongthe other axis (axis of anisotropy) the refractive indices of theparticles 120 and the matrix 140 mismatch. This leads to efficientscattering of light polarized along the preferred axis (x-axis), whichis parallel to the alignment of the nanorods. Along the orthogonal axes(e.g. y-axis), the indices of refraction of the particles and the hostmatrix are matched so light polarized along this axis is transmittedthrough the film and trapped by total internal reflection. Thus, in thisconfiguration, the refractive indices' mismatch exists for light alongthe polarization axis while for the orthogonal axis the refractiveindices are matched. The indices of refraction of the host matrix andthe dispersed particles differ by less than 0.03 in the matcheddirection, more preferably less than 0.02, most preferably less than0.01. The indices of refraction of the host matrix and the dispersedphase preferably differ in the mismatch direction by at least 0.03, morepreferably at least 0.1, most preferably at least 0.2.

In both configurations as described above, light components polarizedalong the polarization axis of the aligned nanorods scatter due to theindex mismatch between the dispersed particles and the host matrix.Light polarized perpendicular to the polarization axis is transmittedthrough the film due to index matching between the two phases. Thisleads to efficient out-coupling of polarized light from the polarizedlight source. The dispersed phase may include anisotropic scattererssuch as polymer fibers or elongated polymer particles, or,alternatively, inorganic anisotropic shaped particles. In someembodiment, the nanorods can be embedded inside the anisotropicscatterers.

As indicated above, various polarized scattering techniques are known.These are described for example in the following patent publications:U.S. Pat. No. 5,825,543, WO 2008/027936, U.S. Pat. No. 7,278,775, andU.S. Pat. No. 8,033,674. According to these techniques, theconcentration of scatterers is preferably high, i.e. 5 to 15% by volumeand more preferably 15% to 30%, and the refractive index mismatch in therequired plane is at least 0.07. In such films, for light incident ontothe film, the angle of refraction at the interface with the film islower than the critical angle for total internal reflection (based onSnell's law). This means that a light path through the film fornot-scattered or weakly scattered light is in the order of the filmthickness (few tens of microns). For this reason, the concentration ofscatterers is generally relatively high and even in the case of highlyscattered light it is within a multiple of the film thickness.

Some characteristics of light scattering, refraction and trapping withinthe film are exemplified in FIGS. 4A-4C. FIG. 4A schematically showstransmission of light through a film/structure 400 and FIGS. 4B and 4Cshow propagation of light components emitted from a nanorod embeddedwithin the film/layer. FIG. 4A shows a light-ray 401 travelling in air(n₁=1) and impinging on the film (n₂) at an angle θ₁; as shown, thelight-ray is refracted at an angle θ₂ upon entrance to the film. Thisrefracted ray travels within the film having a refractive index n₂, andis refracted again at the interface while being transmitted out of thefilm to propagate at an angle θ₁ into the air. Based on Snell's law, inthe general case, when refractive index of the film is higher than thatof the surroundings, all the incident rays are refracted at an anglesmaller than the critical angle for total internal reflection into thefilm. FIG. 4A also shows a cone of angles 404 indicating the possibleangular range for light-rays entering the film from the surroundings.Light-rays propagating inside the film in these angles are not confinedand will out-couple away from the film (assuming parallel interfaces).All the rays inside the cone will not undergo total internal reflectionand therefore their path within the film is of the magnitude of the filmthickness h (the lower limit is h and the upper limit is ˜h/cos(

_(c)), e.g. ˜1.34*h for typical polymers with refractive index of 1.5).

FIGS. 4B and 4C illustrate propagation path of light components emittedby a nanorod located within the film. These figures exemplify theoptically active structure (film) 405, which includes an internal sourceof emission formed by nanorods embedded therein. In other words,emission (e.g. red, green and possibly blue color wavelengths)originates from the emitting nanorods embedded in the optically activestructure. Therefore, emitted light rays 410 from the nanorods impingingthe film-air interface at various angles being above and below thecritical angle for TIR. Light components reaching the interface belowthe critical angle for total internal reflection are extracted from thefilm. However, due to the nature of the emission from the nanorods, alarge portion of light rays, such as ray 430, impinge upon the interfaceat angles larger than the critical angle for total internal reflectionand are trapped within the film. Thus, a significant portion of theemitted light might be trapped within the film, and suitable extractionelements may be required to optimize operation of the film. As indicatedabove, the present technique and optically active film/layer usedprovide suitable configurations of scatterers or surface features forefficient light extraction to prevent loss of energy.

Generally, a majority of the light rays emitted by nanorods aresubjected to total internal reflection, which means that the path lengthtaken by most of the light rays is of order of ˜1-1.5 of the length ofthe film L. Therefore, the concentration of scatterers may besignificantly reduced since the probability of scattering isproportional to the path length multiplied by the density of scatterers'distribution within the film. Thus, for light-ray propagating inside thefilm along a path having length of the order of the length of the film(L) sufficient light scattering may be achieved with concentration ofscatterers below 5% by volume, and more preferably below 1% by volume,and even more preferably below 0.5% by volume.

Alternatively, the concentration of the scattering particles couldremain high, while using particles which exhibit a refractive indexmismatch smaller than 0.07 (which as a consequence will exhibit smallerscattering probability on the single particle level). For example, for ascatterers of size of about 1 μm and medium refractive index of n=1.5,the scattering cross section using scatterers of n=1.57 is 2.8 μm².However, with the refractive index of the scatterer reduced to 1.53(Δn=0.03, instead of Δn=0.07), the scattering cross section is reducedby factor of 5, to 0.55 μm² Thus, it should be noted that the lower themismatch in refractive indices between the scatterers and the matrix,the scatterers concentration is higher.

In some embodiments, the scattering particles can be much larger thanthe wavelength of emitted light, e.g. beyond 10 microns in size. In theconfiguration exemplified in FIG. 3, the nanorods and the anisotropicscatterers are aligned parallel to each other within the opticallyactive structure. For this configuration, since the emission patternfrom the nanorods is anisotropic (similar to dipole radiation pattern),the probability of scattering and interaction of the nanorods' emissionlight rays with the scattering particle is increased. This isexemplified in FIG. 5A illustrating interaction of light emitted from asingle nanorod 1310, embedded in the optically active structure 1300,with a larger anisotropic scatterer 1330 aligned substantially parallelto the nanorod. Radiation output from the nanorods is generallydipole-like in spatial distribution. Thus, light emitted from thenanorods 1320 and propagating within the layer may have a largerprobability of scattering by the scatterer 1330 due to its larger crosssection in this direction (K X d). This effect is prominent for lowscatterers' concentration (e.g. below 1% by volume). In the oppositecase, where the nanorods and the anisotropic scatterer are orientedperpendicular to each other, is illustrated in FIG. 5B showing theinteraction of emission from a single nanorod 1360 embedded in theoptically active structure 1350 with a larger anisotropic scatterer 1380which is aligned perpendicular to the nanorod. The dipole radiation 1370from the nanorods 1360 has a smaller probability of scattering 1390 bythe scatterer 1380 since its cross section is smaller (˜π(d/2)²)).Similarly for the ‘parallel’ configuration, this effect is prominent forlow scatterers' concentration (e.g. below 1% by volume).

Reference is made to FIG. 6 illustrating an example of a display device200. The device 200 is generally similar to that disclosed in theabove-indicated publication WO 2012/059931 assigned to the assignee ofthe present application, and utilizes a backlight illumination unit. Thebacklight unit includes a pumping light source 204 and an opticallyactive structure 202. The system 200 may generally be separated into adisplay section 201 (pixel arrangement) configured for providing desiredspatial modulation of output light to thereby display an image toviewers, and a backlight section 203 configured to produce substantiallyuniform and white/polychromatic illumination. The optically activestructure 202 is preferably configured for polychromatic emission at adesired color temperature. The pixel arrangement 201 may be configuredas an LC panel 205, associated with polarizer 206 at the output of theLC panel. The display device 200 may also include a color filter 216.Additionally, the display device may generally also include anadditional polarizer 207 located at the output of the lighting sectionand is generally provided to obtain a clean polarization state, i.e.block light components of unwanted polarization state which may begenerated by the nanorods due to misalignment and due to the fact thenanorods emit light at a finite polarization ratio. The display device200 may also include a diffuser 208 which spatially homogenizes lightdistribution, and, may be optically attached to the optically activestructure 202; a brightness enhancement film (BEF) 210, or a reflectivepolarizer (e.g., dual brightness enhancement film, DBEF) 212 which maybe used to improve brightness by recycling the light; and may include areflector 214 appropriately accommodated for re-circulating some of thepumping light coming from the light source and other elements.

It should be noted that the diffuser 208, BEF 210, light source 204 andreflector 212 films may or may not be attached to each other and/or toany other element of the display. It should also be noted that thediffuser 208, when optically attached to the active layer 202 may beconfigured for light extraction from the layer by introducing additionalscattering to light components within the active layer 202 and vary thecorresponding angle of propagation.

The above-described display device utilizing emitting nanorods in abacklighting unit can be modified to increase the emission of thepolarized light and improve its out-coupling from the optically activestructure. In some embodiments, the optically active structure 202 isplaced above a lightguide with the pumping source (e.g., Blue LEDs)being coupled to the lightguide at its edges (i.e. edge-illumination).The pumping light is out-coupled from the lightguide and impinges uponthe polarized light source. In another realization, the optically activestructure 202 is optically coupled/attached to the lightguide by anindex-matching layer (for example an optical adhesive with refractiveindex of 1.4-1.5). This is exemplified in FIG. 7 showing a schematicdrawing of the polarized backlight using nanorods films as colorconverters. As shown, a pumping source (e.g. blue LED) 500 is used as aside illuminated light source coupled into a lightguide 505. Thenon-polarized pumping light passes through the lightguide and propagatestowards a polymer layer containing aligned nanorods (e.g. aligned bystretching) defining an optically active structure 515 through anintermediate index-matching layer 510. The nanorods layer is sealed in abarrier film 520 protecting the structure 515 from damage, e.g. moistureand oxygen damage. The layer 515 includes aligned nanorods configured toemit substantially polarized light in selected wavelength ranges, e.g.green and red light, while allowing partial transmission of the blueexcitation source. It should be noted that the optically activestructure 515 may be configured to also emit light in wavelength rangeassociated with blue color, and additional selected ranges to providedesired color temperature. In such configuration the pumping light maybe of blue, violet and/or UV spectra and additional one or morewavelength selective filters may be used.

Polarized light emitted by the optically active layer 515 passes througha polarization preserving diffuser 530 to improve color and brightnessuniformity (intensity distribution) of the backlight before reaching theliquid crystal. Additionally, a reflective layer 540 may be placed onthe rear surface of the lightguide 505 and assist in recycling of lightthat might be emitted backwards from the polymer layer containingaligned nanorods (e.g. aligned by stretching) 515. The emitted polarizedfrom the backlight unit is passed onto a bottom polarizer (optionally)and then to a liquid crystal cell 550.

This configuration of the backlight unit is preferably configured toeliminate air gaps between the lightguide 505 and the optically activestructure 515, enabling increased out-coupling of the pumping light fromthe lightguide into the optically active structure 515 (due todiminished total internal reflection). This configuration significantlyincreases the intensity of the excitation light within the opticallyactive structure, resulting in significant increase in the emission fromthe nanorods. Additionally, the backlight unit, as well as the displaypart of the system, are configured to maintain polarization of lightpassing therealong. To this end, the selection and arrangement of thedifferent layers, as shown in FIGS. 6 and 7, and interfaces between themis provided in accordance with polarization transmission characteristicsof each layer. The layers of the display system are preferably orientedsuch that axis of birefringency (if exists) of the different layers isaligned with the direction of polarization of the emitted light orperpendicular to the direction of polarization of the emitted light.This is in order to eliminate, or at least significantly reduce,rotation of light polarization due to variation in optical path causedby unwanted axis of birefringency.

According to some other embodiments/configurations of the backlight unitof the present invention, the optically active structure is directlyattached to a liquid crystal cell. This is exemplified in FIG. 8illustrating an optical stack for use in a display device. In thisexample, a pumping light source (e.g. blue LED) 600 is used as a sideilluminated light source optically coupled into a lightguide 610 fortransmitting the pumping light towards the optically activestructure/film. Non-polarized pumping light 615 passes through thelightguide 610 and propagates towards a polarization preserving diffuserfilm 620, which may be located between the lightguide and the opticallyactive film 630 to improve the uniformity of the pumping light. This isto optimize intensity distribution of the pumping light and thereforealso of the emitted light. In this example, an optically activestructure includes aligned nanorods layer 630 that is sealed in abarrier film 640, which protects it from moisture and oxygen damage. Thepumping light 615 excites the nanorods layer 630 which emits polarizedlight 635 of one or more wavelength ranges, typically different from thepumping light, (e.g. green and red light). Generally, a portion of thepumping light 615 may be transmitted through the optically active layer630 and may be used as output of the illumination unit. Alternatively,the transmitted portion of the pumping light may be blocked by awavelength selective filter layer, e.g. in the case of UV pumping light.The barrier film 640 is attached directly to a bottom polarizer 655 andliquid crystal cell 660 by an index-matching attachment layer 650.Additional color filter layer 670 may be located upstream or downstreamof the liquid crystal layer 660. The color filter layer 670 includesarray of wavelength selective filters aligned with pixels of the liquidcrystal layer 660 to enable color variations between different pixels ofthe display. Also, some configurations of the optical stack may utilizean additional diffusive layer 680 located downstream of the liquidcrystal layer 660, but may be upstream or downstream of the color filterlayer 670 (when used). The additional diffusive layer 680 is generallyused to improve display quality by providing uniform illumination ofeach pixel of the device. Backward light recycling may be provided by areflective layer 570 that may be placed on the rear surface of thelightguide 610 to redirect light emitted backwards from the opticallyactive layer 630. Thus the reflective layer 570 increases efficiency ofillumination by preventing, or at least reducing, losses caused by lightcomponents propagating in a backward direction.

Thus, in this case, the pumping light 615 (e.g. blue light) is directedby the lightguide 610 and passes through the polarization preservingdiffuser film 620 to improve the light uniformity and deflect the lightoutput from the lightguide in the forward direction. The polarized whitelight 635 from the polarized light source 630 passes through theindex-matching layer 650 directly into the bottom polarizer 655 of theliquid crystal 660 cell. The reflective layer 570 is preferably placedin the rear of the lightguide 610 in order to recycle light emittedbackwards from the nanorods layer 630.

It should be noted that all the above-exemplified configurations andtechniques are generally aimed at effectively extracting polarizedemitted light from the light emitting structure/layer and directing theemitted light to propagate in a desired direction (towards a viewer).The technique of the invention can similarly be used with a lightemitting optically active structure, regardless of relative locations ofthe optically active layer with respect to a lightguide, if any. Itshould be noted that the pumping light may be directed at the opticallyactive layer through propagation within a dedicated lightguide as wellas by free propagation of light. Additionally, the optically active filmmay generally be located at the edge of a lightguide, when used, or ontop of a lightguide.

a lightguide for the pumping light, while being configured forscattering of the emitted light to thereby reduce trapping thereof andprovide efficient illumination in the desired wavelength ranges. Thus,according to some embodiments of the present invention polarizedbacklight can also be obtained by side-lit illumination architecture,for example, by placing a polymer layer containing aligned nanorods(e.g. aligned by stretching) in proximity to a pumping light source(edge-lit), between the pumping source and the lightguide edge, asdescribed in the earlier patent publication of the same assignee, WO2012/059931.

This configuration is exemplified in FIG. 9, showing schematically sucha polarized backlight unit using a nanorods-containing strip 730 (withaligned nanorods) placed on the edge of a lightguide 710. A pumpinglight source 700 is used as an edge-illuminating light source. Pumpinglight from the light source 700 impinges on the strip 730 (acting asoptically active layer/film) and excites the nanorods to emit light ofone or more predetermined wavelength ranges. The nanorods-containingstrip 730 is located between the pumping light source 700 and alightguide 710 and is configured such that light emitted by the nanorodsin the strip 730 is output from the strip to be coupled into thelightguide 710. The nanorods film 730 is preferably sealed in atransparent barrier medium (glass or polymer based) 740 configured toprotect it from damage, e.g. oxygen and/or water or additionalmaterials.

The polarized light from the nanorods strip is coupled into thelightguide 710 and propagates within the waveguide to be out coupledtowards the optical stack. The emitted polarized light may beout-coupled utilizing a grating or grating-like pattern created on thewaveguide, scatterers located within the waveguide, as described abovewith reference to FIGS. 2 and 3, and/or by a polarization preservingdiffuser 780 attached to the waveguide. Such polarization preservingdiffuser 780 may anyway be used to improve color and brightnessuniformity of the backlight before reaching a bottom polarizer(optional) of a liquid crystal cell 770. Similarly to the example ofFIG. 8, a reflective layer 570 is preferably placed on the rear surfaceof the lightguide 710 to recycle light. As shown, the lightguide 710 maypreferably direct and out couple emitted light, and in some cases aportion of the pumping light, at one or more wavelength ranges, threesuch wavelength ranges are exemplified in the figure as 735R, 735G and735B.

In some configurations, the nanorods-containing strip 730 may beoptically bonded to an edge of the lightguide 710 to improve coupling oflight. This may be provided utilizing an index matched adhesive (forexample an optical adhesive with suitable refractive index), as well asutilizing suitable optical assembly and also by proper design of thecontact point. Generally, the strip 730 is placed on the lightguide withthe alignment axis perpendicular to the lightguide edge, or morepreferably with the alignment axis parallel to the lightguide edge.

The degree of polarization of the light output from the lightguide 710can be improved by using a lightguide made of non-birefringent polymerblends other than the standard injection-molded PMMA lightguide. Suchefficient polarization maintaining lightguide was exemplified by Prof.Koike [optical reviews, Vol 19(6), 415-418 (2012)]. Other techniques forachieving non-birefringent lightguides may be used involving processingmethods that do not cause orientation in the polymer chains, forexample, casting or extrusion molding of the polymer at a slow speed orbiaxial stretching.

In some other configurations, polarized light with narrow angulardistribution can be obtained utilizing a light control film as describedfor example in U.S. Pat. No. 6,746,130, which is incorporated herein byreference with respect to this not-limiting example. More specifically,light from a polarized light source strip is coupled into a lightguidedesigned to out-couple most of the light at large angles instead ofout-coupling of light in the forward direction. A specially designedreverse prism sheet is placed above the lightguide attached to theoutput face thereof. The reverse prism sheet efficiently re-directs thelight output from the lightguide and deflects light propagation path totransmit the light in the forward direction towards the LC cell.According to the present technique, however, the reverse prism sheet maypreferably be based on a non-birefringent polymer substrate.

As indicated above, according to some embodiments of the invention, theoptically active structure, as well as the entire polarized light sourceunit, may be configured as a film stack. In this connection, thefollowing should be noted. Typically, light from the polarized lightsource may pass through a polarization preserving diffuser to improvethe color uniformity and brightness uniformity of the emitted polarizedlight output from the backlight unit. Common backlight diffusers whichinclude scattering particles (such as titanium oxide, barium sulfate,acrylic beads or air voids) embedded in a polyethylene terephthalate(PET) film. Such highly diffusive films induce light scattering thatcompletely depolarizes light passing therethrough. Therefore, to providea polarization maintaining stack, the backlight unit may require the useof polarization preserving diffusers in order to maintain polarizationof emitted light. To this end, the optical stack may preferably utilizesurface type diffusers. Such surface type diffusers are to be carefullydesigned based on surface structures, such as micro-lenses or othersurface features inducing surface roughness to cause scattering of lightpassing through, while eliminating, or at least significantly reducing,polarization rotation and loss due to light scattering. Preferably, thediffuser layer is based on a birefringence-free, or a low-birefringence,polymer substrate carrying the surface scattering features. Such polymersubstrate may be formed of one or more of the following: polyacrylates(for example PMMA), polycarbonate (PC), cyclic-olefin copolymer (COP) ortriacetate cellulose (TAC).

In a typical configuration, the polarized light source (including theoptically active structure) is placed on top of a lightguide configuredfor directing light from a pumping light source to the optically activelayer. Generally the pumping light is coupled to the lightguide at oneor more of its edges (i.e. edge-illumination) and is out coupled from atop face of the lightguide to impinge upon the optically activestructure. Additional optical films, such as prism films, brightnessenhancement film (BEF) or reflective polarizers (such a dual brightnessenhancement film (DBEF)), may be used for efficient light recycling andbrightness enhancement, and are located on top of the optically activestructure/film or upstream.

The optically active structure contains suitable nanorods that typicallyemit light in both the forward and backward direction in response to thepumping light, and generally in various additional directions.Therefore, a highly reflective layer is preferably placed in the backsurface of the lightguide in order to recycle light emitted backwards,and returns it towards the lightguide. Such a reflective film ispreferably configured with reflectance above 95% and more preferablyabove 98%. The reflective film may also include highly diffusive sheetscontaining scattering beads (acryl, titanium oxide or barium sulfate).Alternatively, the reflective film may be configured as a multi-layersheet acting as a specular reflector (e.g. ESR film commerciallyavailable from 3M). Generally, the reflective film/layer may be of anyreflector type. It should however, be noted that the reflective film maypreferably be based on one or more metal layers (for example, silver oraluminum) coated on a substrate (e.g. polymer, plastic).

Generally, the following should be noted in connection to lightredirecting (e.g. reflecting, guiding, scattering) optical elementsconfigured for preserving polarization properties of light as describedabove. Polymer films, generally used for optical stacks, may or may notbe configured with certain level of birefringent properties. Part of thebirefringence is typically an inherent characteristic of the polymer,while suitable manufacturing processes may provide for increasing and/ordecreasing birefringence properties. More specifically, some polymerssuitable for use in optical redirecting, e.g. polyethyleneterephthalate, PET, have substantial birefringence properties. Typicalpolymer manipulations may vary a direction of the principle axes and/orlevel of birefringence. For example, stretching of the polymer maychange the refractive index for light polarized parallel orperpendicular to the stretching direction.

The following are some known examples of light redirecting opticalelements (LROE's) suitable for use in the optical stack of a displaydevice or backlighting unit according to the present invention:Brightness Enhancing Films (BEF), which often have prisms shapes on onesurface (by which the BEF is facing an LC layer) of the film thatredirects light to the desired direction normal to the film and also uselight recycling to produce this result; Reverse Prism Films (RPF) thathave prisms facing a lightguide and are used with specially designedlightguides; diffuser films with surface or volumetric featuresconfigured for redirecting light, for example the Light ShapingDiffusers (LSD®, commercially available from Luminit) or Tailoredmicro-diffusers (TMD®, available from WaveFront Technologies);lightguides that redirect light usually entering at an edge andredirecting it in the direction normal to the surface of the lightguide.It should be noted that the term LROE, or light redirecting opticalelements as used herein should be interpreted broadly referring to anyoptical element that may be used in an optical stack and is configuredto vary light propagation therethrough. As indicated above suchvariation may include reflection, refraction and diffusing of light aswell as absorption and re-emission of light components.

It should however be noted that generally LROE's, or any other layer ofthe optical stack, having birefringent properties might cause variationin polarization of light passing therethrough. More specifically, if thebirefringent properties of LROE's are not homogenous with respect toorientation of the principle axes relative to the direction ofpolarization of emitted light. It should be noted that in theconventional, commercially available, LC-based display devices,homogeneous birefringency of the layers are of low importance as thebacklight in not specifically polarized, but rather being polarizedusing one or more polarizers. Thus, a general existing LROE assembly istypically not suitable for use in polarization maintaining film stacksfor the purposes of the present invention, since the polarization stateand/or orientation of light passing through layers thereof may bechanged.

The present invention provides a novel configuration of a lightredirecting structure utilizing a specially designed optical elementconfigured to redirect light by refraction of suitable selectedrefractive features. Such refractive features may for example beassociated with surface geometry of the optical elements, as well assuitable scattering particles (in case of diffusive elements). Anappropriately configured optical element is capable of redirecting lightincident thereon with respect to an angular range of light propagation.For example, the optical elements may be configured for redirectinginput light having a large angle of incidence, to output light with asmaller angular range. This effect may be used to increase the lightintensity directed to a smaller viewing angle, so the image can beviewed by the user whose line of sight to the display venter isperpendicular. Alternatively, the opposite variation may be used toprovide wider angular options for viewers. As indicated, such opticalelement may be configured utilizing proper surface geometry/shapevariation of a LROE film. Such light redirecting optical element isexemplified in FIGS. 10A-10B, showing a birefringent redirecting film800 having predetermined surface features to polarization preservinglight redirecting. Such film may for example be configured as abrightness enhancement film (BEF). FIG. 10A is a schematic illustrationof a birefringence re-directing film 800, e.g. configured as BEF. Theredirecting film 800 is configured with a plurality of prism (orprism-like) features 830 on a support layer 840. The surface features830 extend along a predetermined axis selected to be parallel orperpendicular to the optic axis (birefringent optical axis) of thesupport layer 840. FIG. 10B shows a top view of the light re-directingfilm 800 carrying the surface features (vertical lines correspond to thetip of the prism) 830. Also illustrated in FIG. 10B are the directionsof the fast 810 and slow 820 axes of the redirecting film 800, whichmark the directions of the two principal axes of layer 840.

Specifically, it should be noted that the surface features 830 arepreferably configured to extend along a predetermined axis such that thepredetermined axis is parallel to one of the principal axes of layer 840of the redirecting film 800, i.e. to the fast 810 or slow 820 axes. Incased where the material of the surface features has non negligiblebirefringence, the surface features 830 are preferably configured suchthat the predetermined axis of the features is also aligned withfast/slow axes of the surface features material/layer. It should benoted that although FIG. 10B shows the surface features 830 extendingparallel to the slow axis 820 of the redirecting film 800, similareffect is generally obtained with the surface features 830 extendingparallel to the fast axis 810.

Various designs of the prisms 830 angles, width and height, as well asother parts of the LROE 800, may be used to provide the desiredre-direction of light. It should be noted that the surface features 830may be configured as prisms or prism-like features, e.g. “modifiedprisms” having a curved tip replacing the conventional prism sharp tip,or any other type of features providing desired light re-directing. Itshould also be noted that the optical element of the invention isconfigured such that surface features thereof are aligned withpredetermined angular relation in respect to the slow and fast axes ofbirefringency of the supporting layer 840. Generally, LROE film of theinvention may be configured with one or more of the followingproperties:

The LROE film may be configured with “Zero-Zero” birefringenceproperties, i.e. the film material is prepared with no birefringence orwith similar refractive indices for the slow and fast axes (e.g. asdescribed in Koike et al [12]).

The LROE film may be configured from low level birefringence materials,such as TAC (triacetate cellulose) (e.g. as described in [14]).Additional low birefringence polymers may include cyclic olefincopolymers (COP), polymethyl methacrylate (PMMA) or polycarbonate (PC).

The LROE film may be made from a film having differential retardationproperties for light with different polarization along the fast and slowaxes (e.g. 810 and 820 in FIGS. 10A, 10B). The birefringent propertiesare homogenous across the entire film. In particular, the film isaligned such that the preferred axes of birefringence are along thedirection of the elongated feature's long axis or perpendicular to thisaxis.

It should be noted that the LROE may have any one of the above describedconfigurations. Additionally, the LROE may also be configured utilizingan added separate film selected and aligned to compensate forimbalance/angular variations in the birefringence axes alignment.Generally, such compensation film is configured and aligned to balancethe retardation induced by the LROE film with respect to optical path oflight component polarized along the fast and slow axes of the LROE film(light of the ordinary and extra-ordinary polarization states). This isexemplified in FIG. 11, showing an LROE film 900 carrying a surfacepattern and placed on top of an external layer 910 configured forbalancing optical path of light of selected polarization states. Thecompensation film 910 is configured with birefringent propertiespreselected to compensate the LROE birefringence for light passingthrough the combination of the two films. To this end, the film 910 isselected such that when it is used in combination with (i.e. is stackedwith) the LROE 900, the dual-film combination provides zero, or close tozero birefringence, or birefringence that is oriented with parallel orperpendicular alignment with the nanorod direction.

It should also be noted that, although the invention is exemplified withrespect to light re-directing film, suitable birefringency alignment aswell as the use of compensation film as described above might bepreferred for various films/layers in an optical stack according to thepresent invention. It should also be noted that in order to provideenergetically efficient display system (i.e. high brightness of displayutilizing minimal energy), the optical stack is be configured topreserve polarization of light passing therethrough. More specifically,the different layers/films of the optical stack, including the opticallyactive layer, lightguide, LC panel arrangement, diffusive films etc.,are preferably configured to maintain polarization by proper selectionof birefringence properties and alignment of the fast/slow axes withrespect to axis of alignment of the light emitting nanorods. Asindicated above, this may be achieved using the above described“zero-zero”, low birefringence and compensation film, separately, or inany combination thereof. Additionally, it should be noted that in someembodiments, different layers of the optical stack may be configured toact as a compensation film for other layers of the stack.

Thus, generally the light redirecting optical element, or any otherlayer of the optical stack (optically active film, lightguide, diffuseretc) according to some embodiments of the invention may actually beconfigured as a Selective Birefringent LROE (SBLROE), namely, suchfilms/elements do not depolarize light passing through (interacting withit) and do not rotate polarization of light. Such polarizationmaintaining properties are provided by proper alignment of the slow orfast axes of the SBLROE with respect to polarization direction of theelectric field or magnetic field vectors. Thus, as the polarization axisof light is parallel to the fast/slow axis of the SBLROE, thepolarization direction is substantially not rotated, or is onlypartially rotated. The SBLROE optical stack of the invention canadvantageously be used in an LCD backlight module's optical stack.

In this connection, reference is made to FIG. 12 illustrating an LCDdevice 1000 having an active light modulation module 1002 and abacklight unit 1004. The LCD device 1000 is configured as a layeredstack including a reflective mirror 1104, a light source 1204 (e.g.including a lightguide as described above) and a polarized light sourcefilm 1404 (optically active structure). The backlight unit 1004 may alsoinclude one or more diffusive layers located upstream 1604, ordownstream 1304 of the optically active layer 1404. Additionally, thebacklight unit may include one or more brightness enhancement films(e.g. 1704 and 1804), reflective polarizer (e.g. dual brightnessenhancement film DBEF, 1904) as well as one or more wavelength selectivefilters 1914 for filtering out light of undesired wavelength (e.g.pumping light). As also described above, the optically active layer 1404may be attached to a light extraction layer 1504 configured to enhanceout-coupling of light from the active layer 1404. Generally, each of thelayers of the backlighting unit 1004 is configured to preservepolarization of light passing therethrough as described above. In thisconnection the following elements of the display device 1000 aregenerally considered as elements of the class LROE: reflector 1104,lightguide 1204, diffusers 1304 and 1604, light extraction layer 1504,BEF films 1704 and 1804.

It should be noted, although not specifically shown, that the backlightunit 1004 may also include one or more barrier films configured toprovide suitable protection to selected layers, e.g. the opticallyactive layer 1404, light source 1204, etc.

It should also be noted that, in some embodiments, the backlight unit1004 may include two or more LROE's arranged in a cascade fashion, i.e.one LROE placed on top of another LROE. Such cascaded LROE arrangementmay be used to further re-direct light, e.g. to provide wider angulardistribution with respect to what can be provided by a single layerLROE. It should be noted, and also described above, that each LROE ispreferably configured with either zero- or low-birefringence properties(i.e. induced reduced retardation to light passing therethrough), and/orhave its birefringent optical axis either parallel or perpendicular tothat of the other LROE and to the polarization direction of the emittedlight (alignment axis of the nanorods).

Further, as also indicated above, such zero- or low-birefringence, aswell as proper alignment of the fast/slow axes in case birefringency isan unavoidable property, is generally preferable with respect toadditional elements/layer of the backlight unit 1004 to provideefficient illumination and prevent losses. For example, one or more BEFelements used in the backlight unit 1004 to enhance brightness in one ormore selected wavelength ranges are also preferably configured topreserve polarization of emitted light. The optically active structure1404, being configured as nanorods embedded in a matrix or deposited ona substrate is preferably configured such that the principal axes of thematrix/substrate and barrier layers when used, are aligned with the axisof alignment of the nanorods.

It should be understood that similar alignment with respect tobirefringence properties may be used in other devices employingillumination with polarization properties as mentioned above, includingback illumination and side illumination. In some embodiments, SBLROEused in the unit/device may be in the form of a lightguide configured toreceive polarized light arriving from a predetermined direction and emitit in a predetermined (different) direction. Additional SBLROE ofdifferent forms may be placed on top of the lightguide to provideadditional redirecting properties, such as diffusion and brightnessenhancement.

Thus, the optical stack of the invention, as well as selectedfilms/layers thereof (e.g. SBLROE, diffusive layer, optically activelayer, wavelength filter, etc.), are configured to maintain polarizationof light passing therethrough by eliminating, or at least significantlyreducing birefringence properties, and/or maintain alignment of thefast/slow axes of birefringency with respect to polarization directionof emitted light. Such optical elements and a stack thereof may be usedwith, or as part of, a polarized light source in various applicationsrequiring polarized illumination and maintenance of desired polarizationdirection of output light as well as to increase the light outputdirectionality.

As described above, the optically active structure, backlightillumination unit, as well as the corresponding optical stack, ispreferably configured such that all or some of the optical componentsthereof have no-, or at least reduced, birefringence properties (i.e.induce minimal retardation). Additionally or alternatively, theoptically active structure and additional layers are preferably orientedsuch that a principal axis thereof (i.e. fast or slow axis) is parallelto the direction of polarization of the emitted light. It should also benoted that films/layers having low- or no-optical activity, such asencapsulation film and barrier film (e.g. elements 520 and 640 in FIGS.5 and 6), configured to provide suitable protection to selected elements(e.g. nanorods or optically active layer barrier), are preferably alsoconfigured to maintain polarization by eliminating/reducingbirefringence properties or at least by proper alignment of thefast/slow axes appropriately.

The following is description of the experimental realization of selectedembodiments of the invention, as conducted by the inventors.

EXAMPLE 1

In order to construct a backlight unit, aligned nanorods film was placedon the surface of a lightguide plate (slab) coupled to a pumping lightsource in the form of a blue emitting LED bar (central wavelength 450 nmFWHM=20 nm, edge-lit). A highly reflective sheet based on Silver layercoating was placed in the back surface of the lightguide in order torecycle the light emitted backwards.

The polarized light output from the nanorods layer was directed to passthrough a polarization maintaining diffuser (in this non-limitingexample LSD™ holographic diffuser, circular 30 degrees, available fromLuminit was used) to improve brightness uniformity.

As a light redirecting film, a Reflexite collimating film (RCF™)commercially available from Orafol Europe GmbH was used. A specificbatch had the desired birefringence alignment axes that are along thefacet of the light redirecting films' prism. This film was placedparallel to the nanorods alignment axis or in a perpendicular alignment.In both alignments, the polarized nature from the nanorods film waspartially maintained (a polarization ratio of 1.6 was achieved). Thepolarization ratio is a figure of merit for the polarization performanceof a film, and is measured by placing a polarizer in front of the filmin parallel to the rods alignment axis and perpendicular thereto. Theratio between the two intensities is the polarization ratio. The presentexample shows “partial preserving” of the polarization since thepolarization ratio changed from 3 to 1.6. The SBLROE film provided anincrease of 56% for the blue light with desired polarization and anincrease of 46% for the red component with desired polarization.

This is illustrated in FIG. 13 showing experimental results for theimproved light emitted to the front direction of a waveguide obtained byusing the SBLROE, demonstrated for a single color (red) film. Theseresults correspond to the experiment with the light from the backlightpassing through a polarizer with its transfer axis parallel to the mainpolarization axis of the optically active film. This is done in order toobtain more full polarization properties for the light. It should benoted that although the polarization ratio value of 3 was notmaintained, the polarization ratio level of 1.6 provided additional 23%of light compared to that of using a BEF that completely depolarizes thelight (with polarization ratio equal to 1).

EXAMPLE 2

In order to construct a backlight unit, the aligned nanorods film wasplaced on the surface of a lightguide plate (slab) coupled to a blueemitting LED bar (central wavelength 450 nm FWHM=20 nm, edge-lit). Ahighly reflective sheet based on Silver layer coating (“BL film”,commercially available from Oike) was placed in the back surface of thelightguide in order to recycle the light emitted backwards.

As a light redirecting film, a prism film with 160 um pitch and prismswith angle 90 degrees was used. A specific film with a non-birefringentpolymethyl-methacrylate (PMMA) substrate was chosen (250 micronsubstrate with retardation below 25 nm). This film was placed parallelto the nanorods alignment axis or in a perpendicular alignment. In bothalignments, the polarization ratio received was 2. The LROE filmprovided an increase of 60-80% for the green and red light compared to10-40% increase obtained using prism films which are based onbirefringent polymer substrates. The comparison is shown in FIG. 14.This figure shows the effect of the LROE substrate on the efficiency ofpolarized light extraction. The experimental results show the improvedlight emission (the sum of red and green emitted light componentscompared with this sum for the film's emission without BEF; in thefigure, the y-axis is in percent % units) parallel to the alignment axisof the nanorods and to the front direction of a waveguide obtained by anLROE (prism film) with exactly the same prism structure and differentpolymer material as a substrate. The LROE based on non-birefringent PMMAsubstrate exhibits improved efficiency compared to polycarbonate (PC)and polyethyleneterephtalate (PET) substrates which exhibitbirefringence. It should be noted that all BEF films used herein havethe exact same prisms structure.

Thus, the present invention, as described above, provides for a novelpolarized light source based on polarized emission of an opticallyactive structure including aligned emitting nanorods and utilizes properarrangement of optical elements in the path of emitted light. Thepolarized light source of the invention is configured to maximize theemission of desirably polarized light and its out-coupling from theoptically active structure with maintained polarization state.

1. A polarized light source comprising: at least one optically activestructure comprising a plurality of nanorods configured to emit light ofone or more wavelengths in response to exciting pumping field, saidplurality of nanorods comprising nanorods aligned with a predeterminedalignment axis so as to produce a desired polarization direction of theemitted light; and a light directing assembly comprising one or moreredirecting optical elements in optical path of light emitted from thelight emitting structure, each of said one or more redirecting opticalelements being configured to affect direction of propagation of a lightcomponent interacting with said redirecting optical element whilesubstantially not affecting polarization of said light component, saidlight directing assembly being thereby configured to enhance output ofthe emitted light from the emitting structure by optimizing polarizationand intensity distribution of output light.
 2. A polarized light sourceaccording to claim 1, wherein said one or more light redirecting opticalelements comprise at least one of the following: light recycling opticalelements, reflective layer and diffusive layer.
 3. A polarized lightsource according to claim 1, wherein said one or more light redirectingelements comprise light redirecting elements being aligned with respectto alignment axis of the plurality of nanorods such that a principalbirefringence axis thereof is substantially parallel or substantiallyperpendicular to the alignment axis of the emitting nanorods, therebypreserving the polarization characteristics of emitted light passingthrough the one or more light redirecting optical elements.
 4. Apolarized light source according to claim 1, wherein said one or morelight redirecting elements comprise at least one light redirectingelement configured with low birefringence, defining a retardation factorsubstantially not exceeding 100 nm thereby preserving the polarizationcharacteristics of the emitted light.
 5. A polarized light sourceaccording to claim 4, wherein said at least one light redirectingelement configured with low birefringence has the retardation factorsubstantially not exceeding 50 nm.
 6. A polarized light source accordingto claim 1, wherein said optically active structure comprises a hostmatrix, said plurality of nanorods being embedded in said host matrix,and wherein said optically active structure is configured to enhance theoutput of the light emitted from the nanorods while substantiallymaintaining the polarization characteristics of the emitted light.
 7. Apolarized light source according to claim 6, wherein the opticallyactive structure further comprises a plurality of scattering particlesmixed with said plurality of nanorods and being embedded in said hostmatrix, said scattering particles having different birefringenceproperties with respect to said host material.
 8. A polarized lightsource according to claim 7, wherein the host matrix is formed of anisotropic material composition having zero or relatively lowbirefringence properties of said host matrix, and said scatteringparticles are configured with predetermined non-zero birefringenceproperties.
 9. A polarized light source according to claim 7, whereinthe scattering particles have zero or relatively low birefringence ascompared to a difference between fast and slow axes birefringence of amaterial composition of the host matrix.
 10. A polarized light sourceaccording to claim 8, wherein said scattering particles are configuredto provide refractive index matching for light components havingpolarization orientation perpendicular to said alignment axis whileproviding mismatch in refractive index for light components polarizedalong said alignment axis.
 11. A polarized light source according toclaim 1, wherein the light directing assembly comprises a diffuserlocated in an optical path of pumping light propagating towards theoptically active structure.
 12. A polarized light source according toclaim 11, wherein said diffuser is optically attached to the opticallyactive structure.
 13. A polarized light source according to claim 11,wherein said diffuser is configured with a haze factor between 50% and95%.
 14. A polarized light source according to claim 13, wherein saiddiffuser is configured with a haze factor between 80% and 95%.
 15. Apolarized light source according to claim 1, wherein the light directingassembly comprises at least one diffusive layer located in optical pathof the emitted light propagating from the optically active structure.16. A polarized light source according to claim 15, wherein one of saidat least one diffusive layers is optically attached to the opticallyactive structure.
 17. A polarized light source according to claim 15,wherein said diffusive layer is configured with a haze factor between50% and 95%.
 18. A polarized light source according to claim 17, whereinsaid diffusive layer is configured with a haze factor between 80% and95%.
 19. A polarized light source according to claim 1, furthercomprising an optical pumping assembly configured to generate and directpumping light for exciting said optically active structure, said opticalpumping assembly is configured as a direct backlight pumping lightassembly, directing pumping light onto said optically active structurealong an axis substantially parallel to a light output direction of thepolarized light source.
 20. A polarized light source according to claim1, further comprising an optical pumping assembly configured to generateand direct pumping light for exciting said optically active structure,said optical pumping assembly being configured as an edge-lit backlightpumping light assembly, directing pumping light onto said opticallyactive structure along an axis substantially perpendicular to a lightoutput direction of the polarized light source.
 21. A polarized lightsource according to claim 1, further comprising a lightguide unitconfigured for receiving pumping light from a pumping light source andfor directing said pumping light towards the optically active structure.22. A polarized light source according to claim 1, wherein saidoptically active structure further comprises a protective layerstructure configured to provide at least one of mechanical support andstrain protection and chemical protection to said optically activestructure.
 23. A polarized light source according to claim 22, whereinsaid protective layer structure has at least one of the followingconfigurations: said protective layer structure is oriented such that aprincipal axis thereof is parallel to said axis of alignment of saidnanorods; and said protective layer structure is configured to induceretardation below 100 nm.
 24. A polarized light source according toclaim 22, wherein said protective layer structure comprises at least oneof the following: a transparent encapsulating layer, a barrier layer anda mechanical support film.
 25. A polarized light source according toclaim 1, wherein said at least one optically active structure and saidlight directing assembly defining together an optical stack arrangementcomprising at least two layers oriented such that principal axes ofbirefringence of said at least two layers are parallel to one another,said optical stack thereby substantially preserving predeterminedpolarization properties of light emitted from the optically activestructure and passing through said optical stack.
 26. A display devicecomprising the polarized light source of claim 1, and a spatialmodulating unit configured for varying spatial distribution of outputlight of the polarized light source to thereby provide a displayedimage.
 27. An optical stack arrangement, suitable for use in anillumination device, wherein said optical stack arrangement comprisesone or more layers oriented such that principal axes of birefringence ofsaid one or more layers are parallel to each other; said optical stackthereby substantially preserving predetermined polarization propertiesof light passing therethrough.
 28. The optical stack of claim 27,further comprising at least one diffusive layer configured forincreasing uniformity of light passing therethrough.
 29. The opticalstack of claim 27, further comprising an optically active layercomprising a plurality of nanorods aligned along a predetermined axisbeing parallel to said principal axes of the optical stack.
 30. Abacklight unit for use in a display device comprising a pumping lightsource and the optical stack of claim
 27. 31. A display devicecomprising the backlight unit of claim 30, and a spatial modulating unitconfigured for varying spatial distribution of emitted light to therebyprovide a displayed image.